This month, we focus on the materials used. Previous print articles have addressed the basics of the process (April) and the details of the process chemistry (May).

Nitridable Steels

A variety of steels can be nitrided. Steels with alloying elements such as aluminum, chromium, vanadium and molybdenum are more easily nitrided because they form nitrides that are stable at nitriding temperature. In general, the higher the percentage of alloying elements, the lower the nitriding temperature required and the higher the hardness achieved.

Effects of Alloying Elements

Individual alloying elements (Fig. 24) have different responses to nitriding. These include:

• Aluminum – Strongest nitride former (optimum at approximately 1.5% Al)
• Chromium – Low-alloy chromium-containing steels provide a nitrided case with considerably more ductility (than aluminum-containing steels) but with lower hardness. At high chromium percentages, the effect of chromium is approximately that of aluminum.
• Molybdenum – Forms stable nitrides; reduces the risk of embrittlement at nitriding temperatures
• Vanadium – Forms stable, hard nitrides
• Nickel, copper, silicon and manganese – These elements have little, if any, effect on nitriding.
• Lead – The addition of lead has a slightly negative effect, reducing case depth and hardness.

Properties of Nitrided Steels

Mechanical properties (Table 11) can be enhanced by gas nitriding.

Factors Affecting Case Depth

Users of the nitriding process are interested in the variables that determine the depth and properties (particularly the hardness) of the case produced. The key factors affecting the case depth are: the nature of the nitriding medium; nitriding time and temperature; the amount and nature of nitride-forming elements in solid solution in the steel; and the type, amount and nature of other elements in the steel (C, Ni, Si). In particular:

• The hardness of the case decreases with temperature. Lower temperatures decrease case depth for a given treatment time.
• The amount and nature of the nitride-forming elements in the steel affect the case depth to the extent that the penetration of nitrogen is inversely proportional to the amount of nitrogen that is precipitated (as the alloy nitrides) for a given cycle. The lower the alloy content, the deeper the case for a given time cycle.
• The major function of carbon is to combine with other alloying elements to form carbides, thus removing them from possible reaction with nitrogen.
• It is also known that nickel and silicon together reduce case depth slightly.

A change in nitriding temperature (Fig. 25) from 900° to 1110°F (480° to 600°C) influences the case depth and relative surface hardness. Similarly, a change in nitriding time at constant temperature can result in a variation in surface hardness (Fig. 26).    

The Iron-Nitrogen phase diagram (Fig. 27) provides a “road map” to help determine the type of structures that will be produced in the nitriding process. This figure tells us what happens when nitrogen diffuses into the surface of pure iron.

At nitriding temperatures above 840°F (450°C), nitrogen will dissolve (interstitially) in ferrite (alpha-iron) but only up to a concentration of 0.1% at 1095°F (590°C). When nitrogen content exceeds this value, Fe₄N or gamma prime (y') forms up to a nitrogen content of 5.7-6.1%. This nitride will precipitate at the grain boundaries and preferentially along certain crystallographic planes. As nitriding continues, these nitrides increase in size (as well as quantity) until the entire microstructure has been transformed into a layer of y’. This is the so-called “compound or white layer.”

As the concentration of nitrogen continues to increase, the nitrogen content in the layer also increases. When the nitrogen concentration exceeds 6.1%, y’ nitride starts to change to Fe₃N and Fe₂N, both referred to as epsilon (e) nitride. This transformation starts at the surface (where the nitrogen concentration is greatest), and the y’ layer gradually transforms into epsilon nitride. Meanwhile, the y’ layer is driven deeper (Fig. 28).

Despite the high iron content of the epsilon layer (7.5-9%), the white (compound) layer is nonmetallic in nature. Simultaneously with the increase in thickness of the white layer, the nitrogen diffuses deeper into the steel and new nitrides are precipitated. The zone below the compound layer is called the “diffusion zone” (Fig. 29).    

Since most steels also contain carbon, carbonitrides are formed as well. The nitrides and carbonitrides, in combination with the distortion of the lattice from interstitial atom addition, are quite hard, typically exhibiting ≥67 HRC (900-1100 HV) equivalent. IH

For more information: Dan Herring is president of THE HERRING GROUP Inc., P.O. Box 884 Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: heattreatdoctor@industrialheating.com; web: www.heat-treat-doctor.com. Dan’s Heat Treat Doctor columns appear monthly in Industrial Heating, and he is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.